The current model of melanoma formation is that melanocytes progress from a normal to malignant state by accumulating mutations in key melanoma genes. See, Meier, F., et al. (1998) Frontiers in Bioscience 3:D1005-1010. Melanoma can arise spontaneously, or within a pre-existing nevus or mole. Nevi possess mutations in known melanoma genes and are therefore a risk factor for developing melanoma. See, e.g., Pollock, P. M., et al., (2003) Nat. Genet. 33(1):19-20; Kumar, R. et al., (2004) J. Invest. Dermatol. 122(2):342-348; Chinm L., (2003) Nat. Rev. Cancer 3(8):559-570.
The majority of human melanomas and melanocytic nevi have been shown to have activating mutations in the BRAF, NRAS, C-KIT, or HRAS genes. Furthermore, recent studies have demonstrated that mealnomas fall into genetically distinct groups having marked differences in the frequency of MAP-kinase pathway activation. See, Curtin, J. A., et al., (2005) N Engl J Med. 353(20):2135-47. One category, uveal melanoma, arises from melanocytes within the choroidal plexus of the eye and is biologically distinct from cutaneous melanoma by characteristic cytogenetic alterations. See, Horsman et al. (1993) Cancer 71(3):811. The other category are intradermal melanocytic proliferations, which can be congenital or acquired, and present in diverse ways ranging from discrete bluish moles (blue nevi) to large blue-gray patches affecting the conjunctiva and periorbital skin (nevus of Ota), shoulders (nevus of Ito), and the lower back (Mongolian spot). See, Zembowicz, et al. (2004) Histopathology 45(5):433. These intradermal melanocytic proliferations do not contain either BRAF or NRAS mutations, and thus have a unique eitiology when compared with other nevi and melanoma. See, Ariyanayagam-Baksh S M, et al., (2003) Am J Dermatopathol. 25(1): p. 21-7. Uveal melanomas display MAP-kinase activation (See, Zuidervaat et al. (2005) British J. Cancer 92(11):2032) but typically do not have mutations in BRAF, NRAS, or KIT. Although uveal melanoma is diagnosed in the United States at a rate of 4.3-6 cases per million per year, a previous study of 1250 Caucasians with uveal melanoma found only 17 patients (1.4%) with ocular or oculodermal melanocytosis. See, Gonder J. R., et al., (1982) Ophthalmology, 89(8): 953-60. A potential connection between intradermal melanocytic neoplasms and uveal melanomas is suggested by the fact that nevus of Ota is a risk factor for uveal melanoma and by an overlap in some of the histomorphological features of the two conditions, and the two have been reported to occur together. See, Lopez, M. T., et al., (1998) Am J Dermatopathol. 20:109-110; Singh, A. D., et al. (1998) Opthamol. 105(1):195.
Recently, a large-scale mutagenesis screen in mice identified several dark skin (Dsk) mutants. See, Van Raamsdonk C D, et al., (2004) Nat Genet. 36: 961-968. Some of these mutants had a melanocytic phenotype with a sparse cellular proliferation of intradermal melanocytes resembling blue nevi. The mutations were shown to be the result of mutations in G-protein α-subunits.
G proteins represent a large family of heterotrimeric proteins found in mammals composed of alpha (α), beta (β) and gamma (γ) subunits. See, Wettschureck, N. A. O. S., (2005) Physiol. Rev. 85(4):1159-1204. G-αq, is one of a variety of G-alpha subunits that mediates the stimulation of phospholipase Cβ through the binding and hydrolysis of GTP. See, Markby, D. W., et al., (1993) Science 262(1541):1895-1901. It has been hypothesized that activation of G-αq promotes the survival of melanocytes in the dermis. See, Van Raamsdonk, C. D., et al., (2004). This is consistent with the observation in mice that hyperactivity of G-αq increases the number of melanoblasts, immature melanocytes, migrating in the dermis without increasing their mitotic rate. See, Van Raamsdonk, C. D., et al., (2004).
Germline hypermorphic mutations in Gαq in mice cause dermal hyperpigmentation, without altering epidermal pigmentation. For example, the GnaqDsk1 and GnaqDsk10 mutations are considered to be hyperactive, rather than constitutive because they do not occur in amino acids essential for GTPase activity and remain dependent upon a functioning G protein coupled to the Endothelin B receptor. See, Van Raamsdonk, C. D., et al., (2004). Notably, the GnaqDsk1 and GnaqDsk10 mice do not develop tumors. See, Van Raamsdonk, C. D., et al (2004). However, blocking the GTPase activity through the substitution of critical amino acids can result in constitutive activation. See, Markby, D. W., et al. (1993). For example, a mutation of Q227 in Gαs (Gnas) causes constitutive activity in human pituitary tumors. See, Landis, C. A., et al. (1989) Nature 340(6236):692-696.
Transgenic mice ectopically expressing the G-protein coupled receptor Grm-1 in melanocytes have both dermal hyperpigmentation and large melanocytic tumors. See, Pollock, P. M., et al. (2003) Nat. Genet. 34(1):108-112. Furthermore, injections of constituitively active-Gnaq transformed NIH3T3 cells into athymic nude mice induce tumors within 1 week of injection. See, Kaqlinec G. et al. (1992) Mol. Cell Biol. 12(10):4687-4693.
One mutation in Gnaq has been reported as being present in a melanoma sample. This mutation is described in the Sanger Institute Catalogue Of Somatic Mutations In Cancer (COSMIC) web site on the world wide web at sanger.ac/uk/cosmic. See, Bamford et al (2004) Br J Cancer, 91:355-358. The mutation (Mutation ID No. 18200) described in COSMIC sample id no: 753546 (sample name CP66-MEL) is a missense substitution mutation (1075 G to A) resulting in a conservative amino acid substitution (V359I). There is no teaching that the V359I conservative missense mutation of Gnaq in CP66-MEL, has any effect on Gnaq activity.
The current invention is based, in part, on the discovery that activated Gα subunits resulting from mutations in Gnaq, e.g., mutations that constitutively activate Gnaq, are present in melanocytic neoplasms, e.g., blue nevi, such as nevi of Ota; malignant blue nevi, a rare type of melanoma arising from a blue nevus (see, Granter, S. R., et al., (2001) Am. J. Surg. Pathol. 25(3):316-323); uveal and certain cutaneous melanomas, e.g., lentigo maligna melanoma or melanomas from skin that is damaged by chronic sun exposure (CSD melanoma).